The present disclosure relates generally to components, such as gas turbine engine components including engine combustion system components like combustor liners, and reflective coatings on such components.
A gas turbine engine typically includes a high-pressure spool, a combustion system, and a low-pressure spool disposed within an engine case to form a generally axial, serial flow path about an engine centerline. The high-pressure spool includes a high-pressure turbine, a high-pressure shaft extending axially forward from the high-pressure turbine, and a high-pressure compressor connected to a forward end of the high-pressure shaft. The low-pressure spool includes a low-pressure turbine disposed downstream of the high-pressure turbine, a low-pressure shaft, typically extending coaxially through the high-pressure shaft, and a low-pressure compressor connected to a forward end of the low-pressure shaft, forward of the high-pressure compressor. The combustion system is disposed between the high-pressure compressor and the high-pressure turbine and receives compressed air from the compressors and fuel provided by a fuel injection system. During the combustion process, compressed air is mixed with the fuel in a combustion chamber. The combustion process produces high-energy gases to produce thrust and turn the high- and low-pressure turbines, driving the compressors to sustain the combustion process.
Combustor systems operating at higher temperatures increase engine efficiency. One method of increasing the operating temperature of the combustor is to increase the overall pressure ratio (OPR) in the compression system. As a result, air is discharged from the compressor at a higher pressure and temperature. The OPR of a typical compression system is between 30 and 45, but increasing the OPR to 60, for example, would increase engine efficiency 3 to 6 points. Increasing the OPR also leads to better heat-to-thrust conversion and a 6-9% increase in thrust-specific fuel consumption. However, a higher OPR in the compression system makes it more difficult to cool the walls of the combustion chamber, placing combustor liner durability at risk.
A typical combustor system generates a flame temperature that exceeds the melting point of the metal that lines the combustion chamber. Current methods of reducing the temperature of combustor liners include backside cooling and thermal barrier coatings (TBCs) on the hot gas path surfaces. A typical backside cooling system uses an outer shell having a series of holes surrounding the combustion chamber liner. The series of holes supplies a flow of compressed air to the backside of the combustion chamber liner, cooling the liner to prevent melting. A typical TBC is formed from a ceramic and designed to have low thermal conductivity. TBCs improve combustor liner durability reducing the heat load into the panel.
Total heat load into the panel can be broken down into radiative and convective heat load. Convective heat is generated by the motion of the combustion gases across the combustor liner. Radiative heat is generated by heat radiating from the flame within the combustion chamber. Current methods to increase liner durability only address the convective portion of the heat load. Further, at a higher OPR, a greater portion of the heat load is from radiative and not convective heat. Typically, TBCs only address the convective portion of the heat load because they are largely transparent to the radiative portion at the wavelength of peak flux.